A Ring Laser Gyroscope (RLG) is a rotation sensor that utilizes a beat frequency between two counter-propagating laser beams circulating within a closed laser cavity. The cavity may have a triangular geometry having mirrors at the apexes for reflecting the counter-propagating beams. RLGs are very accurate at medium and high angular rates but become non- linear and eventually insensitive at very low rates. This phenomenon is referred to as mode locking or frequency lock-in and gives an erroneous indication that the RLG is not rotating when in fact it is. The lock-in phenomenon is due to a coupling effect that becomes apparent when two harmonic oscillators resonate at substantially the same frequency. The coupling that occurs in RLGs is primarily due to a small amount of scattering, commonly referred to as backscatter, that occurs at each of the mirror surfaces in the direction of the counter-propagating beams. The amount of backscatter is thus directly related to mirror quality and directly influences the lock-in characteristics of the RLG. Unfortunately, using conventional techniques the mirror quality, and hence the magnitude of the backscatter, is not determined until after a pump and fill process that provides the RLG cavity with a lasant gas.
In U.S. Pat. No. 4,844,615, July 4, 1989, Benoist discloses a lock-in error correction technique that employ a pair of heterodyne detectors to produce heterodyne signals indicative of an interference pattern between the counter-propagating beams. The heterodyne signal is demodulated with a signal indicating the sum of the separate beam intensities to determine the magnitude of coupling between the beams in the RLG.
In U.S. Pat. No. 4,592,656, June 3, 1986, Egli uses a signal generator to modulate the positions of mirrors to phase modulate the backscattered waves an integer multiple of two pi radians.
In general, these patents teach the derivation of RLG lock-in information from the performance characteristics of an operating RLG. What is not taught, and what is thus one object of the invention, is a measurement of backscatter coefficients using a passive RLG cavity.
It is further object of the invention to provide method and apparatus that permits laser cavity mirror quality to be tested prior to the pump and fill process.
It is another object of the invention to directly measure and quantify the magnitude of the backscatter within an RLG cavity to predict the lock-in characteristics of the RLG.